An electric power steering apparatus (EPS) that gives an assist torque (assist force) with a rotatory power force of a motor to a steering system of a vehicle gives a driving force of the motor as an assist to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate an assist torque (steering assist torque), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. A column shaft (a steering shaft or a steering wheel shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control value Vref obtained by performing compensation and so on with respect to the calculated current command value. A steering angle sensor 14 is not absolutely necessary, does not need to be disposed, and further a steering angle can be obtained through a rotational angle sensor such as a resolver or the like connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a torque control section 31 that calculates a current command value by using an assist map and so on and performs compensation and so on with respect to the calculated current command value. A current command value Iref1 calculated in the torque control section 31 is limited a maximum value in a current limiting section 33. A current command value Irefm limited the maximum value is inputted into a subtracting section 32B which is subtracted a motor current detected value Im.
A deviation I (=Irefm−Im) that is the subtracted output of the subtracting section 32B is inputted into a current control section 35 that performs PI (proportional and integral) controls and so on, and a current-controlled voltage control value Vref is inputted into a PWM control section 36 which calculates a duty ratio in synchronous with a carrier signal CF. A motor 20 is PWM-driven through an inverter 37 as a driving section. The motor current detected value Im of the motor 20 is detected by a motor current detector 38, and is fed back to the subtracting section 32B.
A rotational angle sensor 21 such as a resolver is connected to a motor 20, a motor angle θ is outputted from the rotational angle sensor 21.
In such an electric power steering apparatus, in a condition of a steering holding and a low-speed steering, the electric power steering apparatus generates a micro vibration, by which a driver might feel an uncomfortable feeling. As one of the causes, a high frequency noise of a motor angular acceleration signal could be found.
An electric power steering apparatus that is easily tuned up and is intended to suppress a brake-judder and a shimmy and provides safe and comfortable steering performances by performing a signaling process of a road-surface and an external disturbance and so on in a range of high frequencies as a compensator utilized the motor angular acceleration for such a high frequency noise, is proposed in Japanese Patent No. 4715212 (Patent Document 1).
The electric power steering apparatus disclosed in Patent Document 1 has an SAT feedback section that estimates an SAT (Self Aligning Torque) by inputting a motor rotational angular velocity, a motor angular acceleration, a steering assist force and a steering signal. Then, the obtained SAT value is added to a steering assist command value through a transfer function section including a high pass filter (HPF) and a gain section, and the characteristics of the HPF and the gain section vary in sensitive to the vehicle speed signal.